My objective is to explain the mechanisms of cellular determination which underlie embryonic development. An integrated series of light and electron microscopic experiments on Drosophila embryos will ask (1) how a cell arrives at a specific state of determination as a function of its position within the embryo; and (2) how this state is passed on to its descendants. In Drosophila embryos the nuclei divide with near synchrony during the first 13 mitotic cycles. This synchrony is lost in the 14th cycle, when the cells become determined. Then clusters of synchronously dividing cells are distributed over the embryonic surface in a reproducible, bilaterally symmetric pattern. The cells of many such "mitotic domains" exhibit specific morphogenetic behaviors, distinct from those of cells in adjacent domains. Thus this mitotic partitioning of the embryo appears to be an early manifestation of cell determination. This hypothesis will be tested by study of the fate and lineage restraints of cells in individual domains. The diffusion pattern of injected dyes will be used to determine the extent of intercellular communication channels within and between mitotic domains. Electron microscopy will then be used to examine the transcriptional events which underlie this determination. Studies of embryonic chromatin have suggested that DNA replication temporarily perturbs the termination signals of nonribosomal transcription. This perturbation appears to allow active RNA polymerase II molecules to "read-through" the usual termination sites and continue into adjoining DNA sequences. Such replication-dependent transcriptional read-through could activate previously unexpressed control genes and cause the stepwise passage of dividing cells through a succession of determined states. If replication-dependent read-through transcription of long stretches of non-coding DNA in shown to be a general mechanism for activating new genetic loci during development, this could explain why chromosome breakage in dividing tissues results in a high yield of developmental lethals and cancers.